1. Field of the Invention
The invention relates generally to spectrometers, and more particularly to an apparatus for narrowband radiometry and for real time detection, recording, and display of mineralogic spectra in the form of radiated and reflected emissions for airborne geological investigations.
2. Description of the Prior Art
An airborne system for geographical surveys of the earth's surface using a high spectral resolution remote sensing instrument is described by G. Vane, et al in "Airborne Visible/Infrared Imaging Spectrometer (AVIRIS)," JPL Pub. 87-38, Nov. 15, 1987, which is hereby incorporated by reference. The AVIRIS is a 224-channel instrument utilizing the spectral range of 0.4 um to 2.45 um in contiguous spectral bands approximately 10 nm wide. Individual spectra of several minerals were successfully extracted from the AVIRIS data. The AVIRIS instrument, however, was designed for a high altitude, high speed aircraft and is unsuitable to low speed, low altitude aircraft for commercial exploration. Further, while possessing high resolution, only a limited spectral range was covered. Temperature and vibration limitations necessitated utilizing fiberoptic coupling between the foreoptics and the spectrometers, requiring complex active temperature compensation for focusing the fiberoptics. While individual channels were capable of high resolution, the outputs of each detector element were multiplexed for each spectrometer, so that continuous data from each detector element is not available. Further, the dynamic range of the system was limited by a ten-bit analog-to-digital converter.
An improved imaging spectrometer was described by W. E. Collins and S-H. Chang in "The Geophysical Environmental Research Corp. 63 Channel Airborne Imaging Spectrometer And 12 Band Thermal Scanner", SPIE Vol. 1298, pp. 62-71, Apr. 16-17, 1990. The GER imaging spectrometer covers 0.4-2.5 um in 63 channels of varying bandwidth. Both visible light and thermal detection is provided. Scanning optics utilize a rotating polygon mirror which scans the ground four times per revolution, allowing a factor of four reduction of rotation speed to scan speed. The spectrometer design provides for permanent registration between channels without adjustment. All 63 detectors have parallel preamplifiers and analog-to-digital converters. No switching or multiplexing of the detector outputs is used. A 12 bit or greater analog to digital converter permits a system providing high dynamic range with no on-board gain adjustment required. Multiple data channels may be displayed in real time during flight and recorded for later data reduction.
The present invention is an improved model of the GER 63 channel scanner with the capability for up to 640 channels over a range of 0.35 um through 20 um.